Fission

Fission is the division of large atoms to produce smaller species, accompanied by the release of energy.

Uranium

Uranium is the heaviest naturally-occurring element. There are heavier elements, called transuranium (or transuranic), but these are all artifically made, and are unstable. Uranium is too massive to have been made by the Sun - instead all the uranium on the Earth comes from the super-nova which created the disk of debris which eventually coalesced into the solar system.

The Earth is dated by paleontologists by the relative abundances of the isotopes of uranium, knowing the respective half-lives. Lead-208 (Pb: Z = 82, A = 208) is a stable isotope at the end of the decay series of thorium-232 (Th: Z = 90, A = 232), involving beta and alpha decays. At least some of the lead on the Earth was once thorium at some point in the past, which in turn may have been formed by the alpha decay of Uranium-238:

The most abundant isotope of uranium is U-238. This isotope has 92 protons and 146 neutrons, and accounts for 99.274% of all naturally-occurring uranium. The next most abundant isotope is U-235, with only 0.720% abundance. The difference in abundances can be understood by comparing the half-lives: the half-life of U-238 is $4.47 × 10^9$ years, while that of U-235 is $7.04 × 10^8$ years.

Uranium-235 is the only naturally occurring fissile isotope, meaning it can spontaneously undergo fission. Uranium-238 can be made fissionable by bombarding it with fast neutrons. U-238 is fertile: in a nuclear reactor it can be transmuted to plutonium-239. the first reactor was the Chicago Pile-1, built by Enrico Fermi in 1942.

Uranium is enriched to make a more useful fuel in a reactor. In cyclotron, uranium oxide is separated into U-238 and the slightly lighter U-235 isotope. Reactor fuel has typically around 20% U-235 (300 times the natural proportion).

Plutonium (Z = 94, A = 239) does not occur naturally, but may be produced by fast neutrons bombarding uranium-238 (Z = 92, A = 238) to produce Neptunium (Z = 93, A = 239), releasing an electron and a neutrino. The Neptunium in turn emits an electron (a neutron changes to a proton) and a neutrino, and converts to plutonium:

Thorium

Thorium is weakly radioactive and has seven naturally occurring isotopes, all of which are unstable to very varying degrees (half-lives vary from 25.5 hours for Th-231 to 14 billion years for Th-232!). Th-232 has a half-life of 14 billion years, so is by far the most abundant thorium isotope in the crust. It is 3 or 4 times as abundant as uranium.

There is a great deal of interest in utilising thorium as a fuel for nuclear reactors, especially in india, which has no uranium resources, but abundant thorium.

When U-233 undergoes fission, it emits neutrons, which can impact another thorium-232 nucleus, causing the decay to start again. The chain reaction would be self-sustaining at a critical mass and geometry. The cycle is similar to that in fast-breeder reactors, which produces highly-fissile Pu-239 from low-fissile U-238. However, thorium is more abundant than uranium, and offers a more sustainable supply, especially since the fissile U-233 it uses can be 'bred' from natural ore thorium. Most uranium reactors are 'burner' type, and its fuel is enriched natural uranium ore.

Thorium also has the advantage that it can be mixed with U-238, and therefore has no weapons-grade potential, which is not the case with U-235 enriched fuel. It also has a higher neutron yield, produces fewer long-lived transuranium elements, and makes better-performing reactor cores.

Thorium needs to be neutron irradiated before it can be used as a fuel, and this presents greater thechnological challenges, which is why it has not been adopted extensively to date.

Thorium-232 undergoes a complex series of transmutations to arrive at stable lead